1. Field of the Invention
The present invention relates to a semiconductor device having a SiC (Silicon Carbide) substrate as its base, and a method for manufacturing the semiconductor device. More particularly, the present invention relates to a semiconductor device and a manufacturing method thereof that reduce the contact resistance at an ohmic junction between the substrate and an electrode.
2. Description of the Related Art
Semiconductor devices that use a 4H polytype or 6H polytype silicon carbide substrate for the semiconductor substrate that constitutes their base are well known. An example of such a semiconductor device is a Schottky diode. “Silicone carbide substrate” may be abbreviated to “SiC substrate” in the present specification. In the present specification, the term “polytype” may be used in place of “polytype structure” for short.
Electrodes are formed on the surfaces of the SiC substrate. Between the substrate and at least one electrode, an ohmic junction is formed. The electrical resistance at the ohmic junction is almost constant regardless of current or voltage. The electrical resistance (contact resistance) at the ohmic junction is preferably small. Hereinafter, the contact resistance at the ohmic junction may be termed simply the “contact resistance”. A semiconductor device with a low contact resistance is disclosed in Japanese Patent Application Publication No. 2003-86802 (Patent Publication 1). The semiconductor device disclosed in Patent Publication 1 has an epitaxially grown 3C polytype formed on the SiC substrate surface layer where the SiC substrate contacts with the electrode. The 3C polytype is able to reduce the contact resistance due to having a lower bandgap than 4H polytype or 6H polytype. The terms “3C polytype”, “4H polytype” and “6H polytype” refer to types of crystal structure. Particularly, “3C polytype” is sometimes called cubic crystal. “4H polytype” and “6H polytype” belong to hexagonal crystal.